Organic light-emitting diodes (OLEDs), as current mode light-emitting diodes, have been more and more applied to high-performance display. With an increase in display size, a traditional passive matrix OLED requires shorter driving time for a single pixel, which needs to increase transient current and power consumption. In the meanwhile, large current will lead to an excessively large voltage drop on an ITO line and an excessively high working voltage of an OLED, so that efficiency of the OLED will be reduced. However, such problems can be solved well in an active matrix OLED in which OLED currents are inputted by scanning switching transistors progressively.
In an AMOLED backplane design, a main problem to be solved is non-uniformity of brightness of pixels. FIG. 1 shows a structure of the most traditional voltage-driven type pixel driving circuit (2T1C) formed by two transistors and one capacitor. A switching transistor TFT1 transfers a voltage of a data signal line DATA to a control electrode of a driving transistor DTFT, and the driving transistor DTFT converts said data voltage to a corresponding current and supplies said current to an organic light-emitting diode OLED. In normal operation, the driving transistor DTFT should be in a saturation region, and supplies a constant current within scanning time of one row. The current can be represented by the following formula:
            I      OLED        =                  1        2            ⁢                        μ          n                ·        Cox        ·                  W          L                      ⁣      ·                  (                              V            DATA                    -                      V            OLED                    -                      V            thn                          )            2      
where, μn is carrier mobility, Cox is a gate oxide layer capacitance, W/L is a width to length ratio of transistor, VDATA is a data voltage, VOLED is a working voltage of organic light-emitting diode OLED and is shared by all pixel units, Vthn is a threshold voltage of driving transistor, and Vthn is a positive value in the case of an enhancement-mode transistor, and is a negative value in the case of a depletion-mode transistor. However, if the threshold voltages Vthn of different pixel units are different, the currents are difference. If the threshold voltage Vthn of a pixel unit shifts over time, the current thereof may vary over time, which results in afterimage. Moreover, different working voltages of the organic light-emitting diodes OLED due to non-uniformity of the organic light-emitting diodes OLED may also lead to different currents.
To solve the above problems, a threshold compensation unit is provided in the pixel driving circuit so as to compensate for the threshold voltage of the driving transistor DTFT. Nevertheless, the switching transistor TFT1 may still have a problem of electric leakage, which may cause a gate voltage of the driving transistor DTFT to vary when the data voltage changes, so that brightness of the organic light-emitting diode OLED changes over time, thereby resulting in flicker.